Mid-holocene climate change over the Maritime Continent : impact, attribution and mechanisms

Abstract

Speleothem oxygen stable isotope (δ18O) records suggest that precipitation over the Maritime Continent changed significantly during the Mid-Holocene, a time interval about 6,000 years ago. These precipitation proxy datasets show that climatic conditions over the northern part of the region (e.g., Borneo) became wetter, but drier over the southern part (e.g. Flores island). Such changes were hypothesized to be caused by the northward migration of the Inter-tropical Convergence Zone (ITCZ) and/or the weakening of the El Nino Southern Oscillation (ENSO). These two phenomena are mainly driven by changes in the incoming solar radiation (insolation) and also responding to the feedback of sea surface temperature (SST). However, the exact mechanisms through which the two physical quantities contributed to changing the precipitation over the Maritime Continent are still debated, and hence they are investigated here using General Circulation Models (GCMs). The Community Climate System Model version 4 (CCSM4) attributes the higher precipitation in the Mid-Holocene to higher annual variability in its monthly climatology, especially over Borneo. The robustness of the Mid-Holocene annual precipitation signal is further confirmed by its persistence in climate experiments without the ENSO, using the Community Atmospheric Model version 4 (CAM4). The maxima of the change in precipitation monthly climatology fall within the July-October period, and they concentrate over the large islands of the Maritime Continent. The Mid-Holocene higher landmass precipitation is caused by stronger insolation rather than SST feedbacks, as suggested by the numerical experiments using CAM4. The Mid-Holocene stronger insolation increases landmass surface temperature and triggers stronger convection. Then, the low-level southerly wind converges over the landmass and increases humidity, which further strengthens convection over the landmass, suppresses oceanic convection over the South China Sea, and weakens the local Hadley cell.